This invention relates generally to electrical systems which are integrated in a semiconductor chip, and more particularly the invention relates to systems for signal processing such as telecommunications systems and to gyrator filter elements and transconductance devices used therein.
FIG. 1 is a functional block diagram of a wireless receiver for recovering transmitted data. Typically, the receiver includes an antenna 10, filter 12, balan 14, impedance matching network 16, and low noise amplifier 18 for receiving and preparing a signal at a transmission frequency for a frequency down converter shown generally at 20. Down converter 20 includes phase quadrature mixers 21, 22 driven by a phase lock loop synthesizer 23 which reduce the frequency of the received signal and pass the phase quadrature mixer outputs to low pass frequency filters 24, 25. The filtered signals are then passed to mixers 26, 27 which step the filtered signals to an intermediate frequency (IF) for further processing and signal recovery by demodulation of the IF signal as assumed at 28.
The system of FIG. 1 is a fabricated in a single semiconductor chip as an integrated system on a chip. An operating standard, known as Bluetooth, has been established for wireless telecommunications systems on a chip which has stringent operating requirements as illustrated in FIGS. 2A, 2B. The maximum usable signal level the receiver shall operate at will be better than −20 dBm and the reference sensitivity level referred to is −70 dBm.
The charts of FIGS. 2A, 2B show the two-tone requirements for Bluetooth compliance. The reference sensitivity performance, which is a Bit Error Rate (BER)+0.1%, shall be met with a wanted signal at a frequency f0 with a power level 6 db over the reference sensitivity level in the presence of the following: A −39 dBm signal at f1 and a Bluetooth modulated signal at f2 of −39 dBm, where f0, f1, and f2 are defined as f0=2f1−f2 and f2−f1=n*1 mhz, where n can be 3, 4 or 5. System simulations have shown that to achieve a BER of less than 0.1% requires a signal to noise ratio of 20 db (worst case), thus to be Bluetooth compliant in the presence of out of band blockers, the third order in a modulation shall be −64−20 dBm=−84 dBm.
System simulations have shown that channel selection of a Bluetooth signal in the presence of adjacent channel blockers, with powers not exceeding −39 dBM, can be achieved using a fc accuracy of better than 5%.
Filter elements in the system require inductive elements such as shown in FIG. 3A. Since inductors are difficult to realize in integrated circuits and systems, a functional equivalent inductive element, or gyrator, has been devised. As shown in FIG. 3B, the gyrator comprises variable transconductance elements, GM, serially connected with a shunt capacitance, C.
FIG. 4 is a schematic of a known transconductance cell such as discussed Schaumann, simulating Lossless ladders with Transconductance—C Circuits, IEEE Transactions on Circuits and Systems II, Analog and Digital Signal Processing, March 1998, pages 407–410; and Johns and Martin, Analog Integrated Circuit Design, John Wiley and Sons, Inc., 1997, pp 597–600. These prior art references discuss the use of CMOS transconductance using triode (MOS) transistors.
A problem with the use of the known transconductance cell lies in adverse effects of source-bulk voltage (VSB) on MOS transistors used in the transconductance cell. Variations in VSB due to bulk (chip) stray voltages can adversely affect transconductance. Further, control voltage in a gyrator can have a limited dynamic range when using conventional enhancement MOS transistors.